At various times during the life of a well formed in a subterranean formation for the production of oil and gas, it is desirable to treat the well. Such treatments include perforating, gravel packing, fracturing, and acidizing. These treatments generally involve filling the wellbore with a standing or circulating treating fluid. Although high fluid permeability is an important characteristic of a hydrocarbon-producing formation, these treatments may be adversely affected by loss of treating fluid into the highly permeable formations. For example, in an acidizing treatment where it is desired to treat the least permeable formation to improve its permeability, the formation strata having the highest permeability will most likely consume the major portion of the treatment fluid leaving the least permeable formation strata virtually untreated. Therefore, it is desired to control the loss of treating fluids to the high permeability formations during such treatments. During fracturing treatments it is desirable to control loss of the treating fluid to formation to maintain a wedging effect and propagate the fracture. During perforating operations, it is desirable to prevent fluids from entering the formation and damaging the formation. During gravel pack operations, it is desirable to recover the expensive workover fluids from the wellbore after completion of the operation. Therefore, the efficient performance of various treatments of the wellbore require temporarily reducing permeability of formation strata to reduce the loss of treating fluid to these formation strata during treatment. Several fluid loss agents have been developed for use in these treatments.
One type of fluid loss agent which has been used is the oil-soluble, water insoluble compound, such as soaps, gels, waxes and various types of polymers or resins. Water soluble and acid degradable polymeric fluid loss agents are also used. Still another type of fluid loss agent comprises finely divided solids dispersed in the treating fluid, such as crushed naphthalenes and benzoic acid crushed oyster shells, silica flour or guar-coated silica flour or crushed limestone and rock salt.
A particularly useful example of finely divided solids employed as a fluid loss additive is polyhydroxyacetic acid and copolymers of hydroxyacetic acid with other compounds containing hydroxy-, carboxylic acid-, or hydroxycarboxylic acid-moieties. These materials, when ground to a very small particle size, have been found to be very effective fluid loss additives in both fracturing fluids and well completion or workover fluids. When these compounds are used in higher temperature wells &gt;150.degree. F. the polymers hydrolyze and the resulting products are very effective gel breakers. However, at lower temperatures the rate of hydrolysis is too slow to be effective. The hydrolysis rate at low temperatures can be increased by increasing the ratio of lactic acid or other comonomers to hydroxyacetic acid. However, the resulting copolymer is too soft to grind or otherwise to create the very small particle size required to provide good fluid loss control.
Gels are used in the hydraulic fracturing of subterranean formations to carry proppants between the fractured formations to increase the flow capacity of the formation. These gels are damaging viscous materials, such as gels of hydroxypropyl guar and hydroxyethyl cellulose. The process may be applied to new wells to increase productivity or to old wells to restore productivity. In a typical fracturing process, a thickened fluid, such as an aqueous gel or emulsion, is utilized. The thickened fluid increases the fracturing effect and also supports proppant material which is deposited in the fractures created by the process.
When fluid loss additives are included in the fracturing fluid, a gel filter pad comprising fluid loss additive and gel material forms on the surfaces of the wellbore and the fractures created by the process. It is usually necessary to follow the fracturing treatment with gel breaking and/or gel filter pad removal steps. These steps often recover only a small fraction of the potential productivity of the well.
It is desirable to provide a polymer product of hydroxyacetic acid which is effective as a fluid loss agent and also acts as a gel breaker, and in particular, a polymer product which is effective for these purposes at low formation temperatures.
In accordance with this invention, a material which is a fluid loss additive and is also effective as a gel breaker is prepared by subjecting a melted polymer of hydroxyacetic acid with itself or with other hydroxy-, carboxylic acid-, or hydroxycarboxylic acid-containing moieties to high shear and injecting the sheared polymer into an organic liquid which is inert to the polymer to form an organic liquid external dispersion of very fine polymer particles. In one aspect of the invention the melted polymer is combined with the organic liquid and the combination is thereafter subjected to high shear to form a very fine particle polymer dispersion in the organic liquid. In another aspect of the invention, a copolymer is used and the amount of comonomer combined with the hydroxyacetic acid is controlled to provide a copolymer which hydrolyzes at low temperature.